To change the backround bitmap on the logon screen, use Regedt32 to navigate to:
Double-click or Add Value name Wallpaper, a string (REG_SZ) data type. Set it to the full path to your bitmap (.bmp).
NOTE If you move the .bmp file to the %SystemRoot%\System32 folder, you only need the <FileName>.bmp, not the full path.
If you wish to tile the bitmap, double-click or Add Value name TileWallpaper, a string (REG_SZ) data type. A data value of 0 doesn't tile. A data value of 1 tiles the bitmap.
If you wish to stretch the bitmap to fill the screen, double-click or Add Value name WallpaperStyle, a string (REG_SZ) data type. A data value of 0 doesn't stretch the bitmap. A data value of 2 stretches the bitmap. You may not set TileWallpaper to 1 if WallpaperStyle is a 2.
If you don't tile the bitmap and don't stretch the bitmap, it will be centered on your screen, and partially hidden. If you wish to set the location of the bitmap, Add Value name WallpaperOriginX as a string data type, and set it to the number of pixels from the left hand edge of the screen. Add Value name WallpaperOriginY as a string data type, and set it to the number of pixels from the top of the screen.
NOTE: If you set WallpaperOriginX and WallpaperOriginY to 0, it centers the wallpaper. If you meant top left corner, set both to 1.
If the bitmap doesn't fill the screen, you may want to set the backround color. Navigate to:
HKEY_USERS\.DEFAULT\Control Panel\Colors\. Double-click the Background value name and set the string to the RGB value. The default string is 58 110 165. Here is a table of common colors: